J. Org. Chem. 1999, 64, 4247-4254
4247
Synthesis of peri-Cyclobutarenes by Thermolysis of [Methoxy(trimethylsilyl)methyl]arenes Thomas A. Engler† and Harold Shechter* Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 Received June 9, 1998
[Methoxy(trimethylsilyl)methyl]arenes are readily prepared by reactions of chlorotrimethylsilane with (R-methoxy)arenylmethyllithium reagents as obtained from (methoxymethyl)arenes and t-BuLi. The [methoxy(trimethylsilyl)methyl]arenes eliminate methoxytrimethylsilane at 525-675 °C/0.050.10 mm to yield peri-cyclobutarenes as derived from arenylcarbenes. Of importance is the fact that the initial arenylcarbenes generated insert into adjacent peri C-H bonds and/or isomerize to other arenylcarbenes that insert into their peri C-H bonds to give peri-cyclobutarenes. Thus, flashvacuum pyrolysis of 1-[methoxy(trimethylsilyl)methyl]naphthalene (13) at 575-675 °C/0.05-0.10 mm yields 1H-cyclobuta[de]naphthalene (6, up to 39%) in practical quantities. 2-[Methoxy(trimethylsilyl)methyl]naphthalene (23) also affords 6 as a major thermolysis product. At 510 °C/ 0.05-0.10 mm 4-methoxy-1-[methoxy(trimethylsilyl)methyl]naphthalene (29) decomposes to 4-methoxy-1H-cyclobuta[de]naphthalene (31, 46%). Under similar conditions, 2-methoxy-1-[methoxy(trimethylsilyl)methyl]naphthalene (33) converts to 1,2-dihydronaphtho[2,1-b]furan (35, 64%) and naphtho[2,1-b]furan (36, 31%), presumably by insertion of 2-methoxy-1-naphthylcarbene (34) into a C-H bond of its o-methoxy group and then dehydrogenation of the resultant dihydrofuran. Further, 1-[methoxy(trimethylsilyl)methyl]-6-methylnaphthalene (39) pyrolyzes (510 °C/0.10-0.20 mm) to 6-methyl-1-naphthylcarbene (40), which isomerizes in part to 7-methyl-1-naphthylcarbene (49); carbenes 40 and 49 then undergo peri C-H insertion to give 3-methyl-1H-cyclobuta[de]naphthalene (41) and 2-methyl-1H-cyclobuta[de]naphthalene (42) in an 8:1 ratio and a combined yield of 44%. The pyrolytic method is particularly valuable for preparing higher peri single carbon atom bridged arenes such as 4H-cyclobuta[jk]phenanthrene (53, 65%) and 3H-cyclobuta[cd]pyrene (59, 86%). Naphthalenes have been bridged in peri-positions by single carbon atom moieties by (1) photolysis of 8-halo1-naphthyldiazomethanes (1, X ) Br, and 3, X ) I; eq 1),1 (2) pyrolysis of 1(5)- and 2(7)-naphthyldiazomethanes
(Scheme 1), sodium 1- and 2-naphthaldehyde p-tosylhydrazonates, and 5-(1)- and 5-(2-naphthyl)tetrazoles, respectively at 450-700 °C/10-2-10-4 mm,2 and (3) coupling of 1,8-dilithionaphthalene (8, Scheme 1) with dichloromethane and 1,8-bis(iodomagnesio)naphthalene (9, Scheme 1) with methylene bis(toluene-p-sulfonate).3 The above synthetic methods are often unsatisfactory, however, for preparative purposes. Practical syntheses of various peri-methanonaphthalenes (11) by vacuum † Present address: Lilly Corporate Center, Eli Lilly and Company, Indianapolis, IN, 46285. (1) (a) Bailey, R. J.; Shechter, H. J. Am. Chem. Soc. 1974, 96, 8116. (b) Bailey, R. J.; Card, P. J.; Shechter, H. J. Am. Chem. Soc. 1983, 105, 6096. (2) (a) Becker, J.; Wentrup, C. J. Chem. Soc., Chem. Commun. 1980, 190.2c (b) Wentrup, C.; Mayor, C.; Becker, J.; Lindner, H. J. Tetrahedron 1985, 41, 1601.2c (c) The p-tosylhydrazonates and the (naphthyl)tetrazoles decompose to 5 or 7 prior to formation of 6.2a,b (3) Yang, L. S.; Engler, T. A.; Shechter, H. J. Chem. Soc., Chem. Commun. 1983, 866.
Scheme 1
thermolyses of [methoxy(trimethylsilyl)methyl]naphthalenes (10) as illustrated in eq 2 are now described in
detail.4-6 The present method has also been extended to synthesis of various benzannelated derivatives of 11.
10.1021/jo981104j CCC: $18.00 © 1999 American Chemical Society Published on Web 05/26/1999
4248 J. Org. Chem., Vol. 64, No. 12, 1999
1-[Methoxy(trimethylsilyl)methyl]naphthalene (13) is readily prepared (100%, eq 3) by deprotonation of 1-(meth-
oxymethyl)naphthalene (12) with t-BuLi in TMEDA/Et2O at -78 °C and reaction of the subsequent lithio derivative with chlorotrimethylsilane.7 Vacuum pyrolysis of 13 upon volatilization through a packed quartz tube at 650 °C/ 0.05-0.10 mm occurs with loss of methoxytrimethylsilane to yield 1H-cyclobuta[de]naphthalene (6, eq 3) as a major product (39%)8 along with 12 (8%), naphthalene (14, 3%), 1-naphthaldehyde (9%), R-methyl-1-naphthalenemethanol (7%), 1-methylnaphthalene (5%), and intractables. Cyclobutanaphthalene 6 is obtainable, after chromatography of the pyrolysate, as the major component (60-70% by 1H NMR) of a mixture containing 14, 1-methylnaphthalene, and unidentified minor products. This mixture can be used conveniently for preparing derivatives of 6 as demonstrated by nitration with acetyl (4) (a) A portion of these results were communicated: Engler, T. A.; Shechter, H. Tetrahedron Lett. 1982, 23, 2715. (b) For application of this chemistry, see: Jaworek, W.; Vo¨gtle, F. Chem. Ber. 1991, 124, 347. (5) (a) Other methods for generating arenylcarbenes at elevated temperatures for preparing peri-cyclobutarenes are frequently impractical because of the instabilities of aryldiazomethanes and their facile conversions to azines, the low vapor pressures of sulfonylhydrazonate and sulfinate salts, the problems in introducing solids which are not free-flowing into pyrolysis equipment at low pressures, and the inefficiencies in decompositions of aryltetrazoles to their corresponding arylcarbenes.2b,5b-d (b) The principal thermal reactions of aryltetrazoles are conversions to nitriles and hydrogen azide.2b,5c,d (c) Golden, A. H.; Jones, M., Jr. J. Org. Chem. 1996, 61, 4460. (d) Kumar, A.; Narayanan, R.; Shechter, H. J. Org. Chem. 1996, 61, 4462. (6) (a) Photolysis of 2-diazoacenaphthenone in argon at 8 K to 1,8naphthyleneketene and irradiation of 8-hydroxy-1-naphthylglyoxylic acid lactone at -195 °C to give 1H-cyclobuta[de]naphthalen-1-one have been of significance to structural principles and mechanistic theory.6b These transformations as yet, however, are not usable preparatively. (b) Chapman, O. Chem. Eng. News 1978, Sept 18, p 78. (c) Hayes, R. A.; Hess, T. C.; McMahon, R. J.; Chapman, O. L. J. Am. Chem. Soc. 1983, 105, 7787. (7) (a) R-Lithio(aryl)methyl alkyl ethers [ArCH(Li)OR] rearrange readily to lithio alkoxides [ArCH(R)OLi].7b-e Such Wittig rearrangements are retarded at low temperatures by TMEDA. (b) Wittig, G.; Lo¨hmann, L. Liebigs Ann. Chem. 1942, 550. (c) Schafer, H.; Scho¨llkopf, U.; Walter, D. Tetrahedron Lett. 1968, 2809. (d) Scho¨llkopf, U. Angew. Chem., Int. Ed. Engl. 1970, 9, 763. (e) Hoffmann, R. W.; Ru¨hl, T.; Harbach, J. Liebigs Ann. Chem. 1992, 725. (f) That R-lithio(aryl)methyl methyl ethers can be used synthetically in the presence of TMEDA at low temperatures without significant rearrangement was developed4a,7g on knowledge that the migratory abilities of alkyl groups in isomerization of R-lithiobenzyl alkyl ethers in TMEDA/THF/Et2O at -60 °C increase in the order methyl, ethyl, iso-propyl, and tert-butyl (krel 1:40: 162:2080).7c (g) Engler, T. A.; Shechter, H. Tetrahedron Lett. 1983, 24, 4645. (h) Yeh, M. K. J. Chem. Soc., Perkin Trans. 1 1981, 1652. The author found that benzyl methyl ether is deprotonated by n-BuLi/ TMEDA in hexane at -10 °C and the R-lithiobenzyl methyl ether formed is alkylated by 1-bromobutane (80% yield) and adds to nonenolizable ketones. (i) Azzena, U.; Demartis, S.; Fiori, M. G.; Melloni, G.; Pisano, L. Tetrahedron Lett. 1995, 36, 5641. The authors have communicated that R-lithio(aryl)methyl methyl ethers, as generated by metalation of arylmethyl methyl ethers with n-BuLi in THF at -40 °C in the absence of TMEDA, react with various electrophiles without serious complications from Wittig rearrangements. (8) Piptopyrolysis of sodium 1-naphthaldehyde p-tosylhydrazonate at 600 °C/10-3 to 10-1 mm is reported to give 6, 14, and 1-methylnaphthalene in 39%, 15%, and 21% yields, respectively.2b At 700 °C/ 10-3 to 10-1 mm, 5-(1-naphthyl)tetrazole yields 6 (26%), 1-methylnaphthalene (6%), and naphthalene-1-carbonitrile (61%).2b In the present investigation, piptopyrolysis of 5H-(1-naphthyl)tetrazole at 600 °C/0.05-0.1 mm in a quartz tube packed with quartz chips yields naphthalene-1-carbonitrile and hydrogen cyanide essentially totally.
Engler and Shechter Table 1. Effects of Temperature and Pressure on the Conversions to and the Yields of 6 on Pyrolysis of 13 temp (°C)
pressure (mm)
6 % conversiona
% yieldb
13 % recoveryc
490-510 500-510 500-510 560 650 695-700 750 800
0.005 2.0 0.35-0.40 0.07-0.10 0.05-0.15 0.05-0.07 0.05-0.07 3.0
0 6-11 17 11 39 27 4 0
0 9-12 35 32 39 27 4 0
92 35 51 65 0 0 0 0
a The percentage of 6 formed from 13 in a single pass. b The percentage of 6 formed from 13 that decomposed. c The percentage of initial 13 recovered.
nitrate to give pure 4-nitro-1H-cyclobuta[de]naphthalene (15).9 Purification of 6 (>90%) can be effected by fractional distillation, preparative GC, or preparation of its picrate followed by recrystallization and decomposition of the complex on silica gel.
Pyrolysis of 13 is conducted in simple equipment (see Experimental Section) at readily attainable temperatures and pressures. More than 10 g of 13 has been thermolyzed in a single experiment, and the decompositions can be conducted on a larger scale.10 Varying the pyrolysis temperature and pressure greatly affect the conversions and yields of 6 from 13. As shown in Table 1, temperatures of at least 500 °C are required, and the best yields of 6 are obtained at lower pressures (100 10 9 6 4
a The percentage of 53 and 54 from 51 in a single pass. b The percentage of 53 and 54 formed from 51 that decomposed. c The percentage of initial 51 recovered.
bridging in CH2 in 41 and 42 appear at δ 4.76 and 4.70, respectively. Investigation was then initiated of the thermal behavior of benzannelated naphthylcarbenes as generated from methyl R-(trimethylsilyl)arylmethyl ether precursors. The first such carbene studied was 9-phenanthrylcarbene (52) upon synthesis and decomposition of 9-[methoxy(trimethylsilyl)methyl]naphthalene (51, Scheme 8).4,16 Pyrolysis of 51 at 590 °C/0.1 mm gives 4H-cyclobuta[jk]phenanthrene (53) and 4H-cyclopenta[def]phenanthrene (54) in a 9:1 ratio and a combined yield of 72%. The yield of 53 and the ratio of 53 to 54 vary greatly with the thermolysis conditions (Table 3). Indeed, no 54 is obtained when the pyrolysis is conducted at low conversion of 51 to products. In general, purification of 53 is much simpler when the decompositions are effected at lower temperatures. As expected, conversion of 52 to rearrangement product 54 increases with temperature. The major product, 53, is obtained pure by recrystallization and is assigned from its combustion analysis, exact mass, and spectral properties. The important spectral features of 53 are its 1H NMR absorptions at δ 4.80 (s, 2H, CH2), 7.25 (dd, 1H, H-3 on the phenanthrene nucleus, J ) 2 and 6 Hz), 7.31 (s, 1H, H-5, this signal and that of H-3 partially overlap), and 7.5-8.5 (m, 7H, aromatic) and 13C NMR absorption at 46.6 (CH2), as well as the proper number of aromatic C-H and quaternary C signals. Cyclobutaphenanthrene 53 is a sublimable, white crystalline material and forms a bright orange complex with 2,4,7-trinitrofluoren-9-one. Identification of 54 resulted from 1H NMR and HPLC comparison of the (16) Reference 2b reports that piptopyrolysis of 5-(9-phenanthryl)tetrazole at 700 °C/10-3 to ∼5 × 10-1 mm and sodium phenanthrene9-carboxaldehyde p-tosylhydrazonate at 650 °C/10-3 to 10-1 mm each yield 53 and 54 in ∼7% and 1% yields; phenanthrene, 9-methylphenanthrene and 9-phenanthrenecarbonitrile are also formed.
pyrolysis mixture of 53 and 54 with 54 that had been synthesized independently. The mechanisms of rearrangement of 52 are similar to those of naphthylcarbenes. Thus, 53 is formed by cyclization of 52 at C-1 with hydrogen migration. Cyclopentaphenanthrene 54 results (Scheme 9) after at least 20 formal rearrangement steps in the isomerization of 52 to 4-phenanthrylcarbene (55), which then inserts into its C-H bond at C-5. Rearrangement of 52 to 55 and then 54 is also of interest in that carbenic migration through a fused ring juncture occurs.16 Synthesis of 1-[methoxy(trimethylsilyl)methyl]pyrene (57, 77%) from 1-methoxymethylpyrene (56), its thermolysis to 1-pyrenylmethylene (58), and the formation of 3H-cyclobuta[cd]pyrene (59, Scheme 10) were then studied. Indeed, volatilization of 57 through a quartz tube at 520-525 °C at 0.05-0.07 mm produces 59 in 86% yield. Identification of 59 is made from its exact mass and spectral properties including 1H NMR absorptions at δ 5.19 (s, 2H, CH2), 7.50 (s, 1H, H-4 on aromatic ring), 7.62 (d, 1H, H - 2, J ) 6 Hz), and 7.8-8.2 (m, 6H, aromatic) and a 13C NMR signal at δ 53.0 (CH2). Pyrene 59 is a sublimable, white crystalline solid that reacts rapidly with air on standing at room temperature; however, 59 forms an air-stable bright maroon-purple
4252 J. Org. Chem., Vol. 64, No. 12, 1999
Engler and Shechter
Scheme 10
Table 4. Effects of Temperature and Pressure on the Conversions to and the Yields of 59 on Pyrolysis of 57 temp (°C)
pressure (mm)
59 % conversiona
% yieldb
57 % recoveryc
510-530 520-525 520-540 660
0.02-0.05 0.05-0.07 0.05-0.10 0.05-0.20
29 32 31
